Lightweight mirrors are needed for a wide variety of applications, especially in the space and aeronautic industries where overall system weight reduction and precision of optical parts are critical. Using a light weighting process, the mass of a mirror may be reduced by up to 90% compared to a solid blank of the same size and material.
One method for fabricating a lightweight mirror involves creating a lightweight mirror blank consisting of a face sheet and a lightweight support structure such as a low-density honeycomb structure.
When the lightweight blank is optically ground and polished, the local variation in stiffness due to the light weighted structure can cause “quilting” or “print-through”. This is caused by a variation in deflection of the surface based on the pattern of the reinforcing ribs. In other words, the variation in deflection causes a proportional variation in material removal rate creating the quilt-like pattern.
The problem above is usually avoided by making the face sheet thicker. However, this solution adds more mass to the mirror. The problem can also be avoided by grinding and polishing a solid blank and light weighting afterward as described in U.S. Pat. No. 3,753,322 ('332).
Another method claimed by the patent '322 comprises forming at least one optical surface on a solid blank by machining and mechanical polishing, ultrasonically machining cavities underlying the optical face in at least one of the remaining faces of the blank to reduce the weight of the article. The patent '332 also discloses that the optical surface so lightened is to be coated by high-temperature, vacuum-evaporation deposition of aluminum.
The drawbacks of the method disclosed in '332 is that the optical surface still suffers greatly from distortion during the light weighting process. Virtually any polycrystalline or vitreous material, no matter how well annealed, will have some residual internal stress. During the light weighting process, as stressed material is removed, the remaining material distorts, ruining the optical surface.